The present invention relates to a device for receiving acoustic waves in water and more particularly a reception device comprising a plurality of receivers disposed operationally at different depths.
The device of the invention finds applications in the fields of oceanography, acoustic detection or else seismic prospection at sea, for example.
In the field of oceanography it is known to use radio buoys comprising a floating body associated with radio transmitting equipment connected to one or more immersed sensors such as hydrophones. The sensors are supported either by a weighted cable, or by a tube, or a column, which is adapted to remain in a substantially vertical position and is very often anchored to the sea bed. Such buoys are described in, for example, U.S. Pat. Nos. 3,405,558, 3,488,783 or 3,541,498.
Certain seismic prospection operations, by refraction, for example, comprise the use of a radio buoy from which is suspended a sensor such as a hydrophone, adapted for detecting the acoustic waves refracted by the different layers of the subsoil, in response to seismic pulses emitted by a marine source towed behind a ship. The buoy is placed in the water and the ship moves off from it. The seismic source which it tows is actuated periodically. The waves picked up are transmitted by radio to a recording laboratory installed on the ship.
A buoy of this type is described in, for example, U.S. Pat. No. 3,671,928.
The use of one or more free radio buoys left behind a seismic ship has numerous drawbacks.
Because of the currents, it is sometimes difficult to determine their exact positions at the successive moments when the seismic source is actuated. It is also often difficult to locate them, once the seismic prospection operations are finished and when that is possible recovery thereof requires manoeuvers which are sometimes long.
In numerous cases, it is preferred to use radio buoys with a single transmission channel and having inexpensive fairly scratch electronic equipment which may therefore be abandoned at the end of the seismic surveying if circumstances do not lend themselves to their recovery.
Furthermore, radio buoys which are not anchored are subject to the movements of the swell and the sensor elements suspended below are subject to vertical movements which generate background noise. The damping means which are often found associated with the cable supporting the sensor element has only a limited power and contributes to making the radio buoys more complex and more difficult to place in the water.
Another disadvantage of radio buoys related to the preceding one concerns their size especially when they comprise a plurality of spaced sensors, fixed to a relatively long immersed part. It is difficult to move such a buoy from one position to another because it is often necessary to hoist it entirely out of the water.
The device of the invention avoids the above mentioned drawbacks.